The invention relates to a solid state light-emitting device.
Light emitting devices can be used, for example, in displays (e.g., flat-panel displays), screens (e.g., computer screens), and other items that require illumination. Accordingly, the brightness of the light emitting device is one important feature of the device. Also, low operating voltages and high efficiencies can improve the viability of producing emissive devices.
Light emitting devices can release photons in response to excitation of an active component of the device. Emission can be stimulated by applying a voltage across the active component (e.g., an electroluminescent component) of the device. The electroluminescent component can be a polymer, such as a conjugated organic polymer or a polymer containing electroluminescent moieties or layers of organic molecules. Typically, the emission can occur by radiative recombination of a trapped charge between layers of a device.
The energy of the emitted light can correspond to the energy difference between bands, i.e., between the ground state and excited state of the materials. The emitted light has an emission profile that includes a maximum emission wavelength, and an emission intensity, measured in luminance (candelas/meter2; cd/m2). The emission profile, and other physical characteristics of the device, can be altered by the electronic structure (e.g., energy gaps) of the material. For example, the brightness, range of color, efficiency, operating voltage, and operating lifetimes of light emitting devices can vary based on the structure of the device.
In general, the invention features a solid state light-emitting device having a high maximum luminance, a high external efficiency, and a low operating voltage.
In one aspect, the invention features a solid state light-emitting device. The device includes a solid layer, a first inert electrode, and a second inert electrode. The solid layer includes a metal complex and has a first surface and a second surface. The first inert electrode contacts the first surface of the solid layer. The second inert electrode contacts the second surface of the solid layer. The device has a luminance of at least 50 cd/m2 at a potential of between 2.5 and 5.0 V.
In another aspect, the invention features a solid state light emitting device including a solid layer consisting essentially of a ruthenium bipyridine complex.
In another aspect, the invention features a solid state light emitting device including a solid layer. The solid layer includes a non-polymeric metal bipyridine complex and the device has a luminance of at least 50 cd/m2 at a potential of between 2.5 and 5.0 V.
In certain embodiments, the device can have a luminance of at least 100 cd/m2 (e.g., at least 200 cd/m2 or at least 1000 cd/m2) at a potential of between 2.5 and 5.0 V. The device can have an intensity drop to one-half of a maximum intensity at 2.8 V of at least 5 hours (e.g., at least 20 hours, or at least 50 hours). The device can have an external efficiency of at least 0.3 percent (e.g., at least 0.5 percent or at least 0.75 percent).
The metal complex can be non-polymeric. The metal can be ruthenium or osmium. The metal complex can be a transition metal complex, such as a hexafluorophosphate salt of a ruthenium bipyridine complex. In certain embodiments, the metal complex is a bipyridine complex. The bipyridine can have a hydroxymethyl substituent, a C1-18 alkoxycarbonyl substituent, or a hydroxy substituent, or combinations thereof.
Each inert electrode can be selected from a group consisting of indium tin oxide, a metal, such as aluminum, silver, gold, platinum, or palladium, and a conducting polymer, such as polypyrrole. In certain embodiments, the first inert electrode can be selected from a group consisting of indium tin oxide, aluminum, silver, gold, platinum, and palladium. The second inert electrode can be selected from a group consisting of indium tin oxide, silver, gold, platinum, palladium, and polypyrrole.
In yet another aspect, the invention features a method of generating light. The method includes applying a light-generating potential of between 2.5 and 5.0 V across a first inert electrode and a second inert electrode of a solid state light-emitting device, and generating light from the device having a luminance of at least 50 cd/m2. The device includes a solid layer, a first inert electrode, and a second inert electrode. The solid layer includes a metal complex and has a first surface and a second surface. The first inert electrode contacts the first surface of the solid layer. The second inert electrode contacts the second surface of the solid layer. The method can further include the step of applying a charging potential of greater than 5.0 V for less than 30 seconds prior to applying the light-generating potential.
In another aspect, the invention features a method of manufacturing a solid state light-emitting device. The method includes depositing a solid layer including a metal complex onto a first inert electrode, and placing a second inert electrode onto the solid layer, avoiding contact with said first inert electrode. The device has a luminance of at least 50 cd/M2 at a potential of between 2.5 and 5.0 V. The solid layer can be deposited, for example, by spin coating a solution on a surface of the first electrode.
The solid state light-emitting device based on a solid layer of a metal complex exhibits good air stability, high efficiency, and high luminance, and operates at low voltages. In addition, device fabrication and operation are straightforward and simple. In fact, low cost fabrication techniques, such as spin coating, can be utilized to make the devices.
For example, the solid state light-emitting device based on a Ru(II) complex can be activated to high brightness without elaborate film fabrication or charging schemes, such as solvent-swelling. In addition, reactive cathode materials (e.g., Ca, Mg) are not needed, and additional electrolyte materials are not added to the solid layer. In fact, red-orange or red luminance of at least 50 cd/m2 is unprecedented in neat films of non-polymeric emitters at 2.5 to 5V. For example, at 2. 5V, a device based on compound I, shown in FIG. 2, operates with a luminous efficacy of 1.4 lm/W (0.6 cd/A), making the material attractive for flat-panel display applications.
The low operating voltage of the solid state light-emitting device allows an applied bias close to the redox potential of the emitter to be used. The low voltage is sufficient to both induce counter-ion migration and effect the required redox reactions to generate light from the solid layer. At slightly higher voltages, charging times decrease dramatically. By applying a high voltage for a brief amount of time, followed by normal operation at low voltage, the charging time of the device can be significantly reduced. Alternative biasing schemes, such as imposing an alternating current field over a dc bias, can extend device lifetime several-fold at comparable brightness,
Other devices having a multiple layer structure include at least two thin organic layers (i.e., a hole transporting layer, and an electron transporting layer) separating the anode and cathode of the device. This device operates like a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Light emitted from such a layered device depends on the direction of the applied bias and the electrode materials, The single solid layer device of the invention can emit light under any bias direction. In addition, the single solid layer device of the invention can employ a variety of inert electrode materials.
Other features or advantages of the present invention will be apparent from the following detailed description and also from the claims.